1,3,5-Triferrocenyl-2,4,6-tris(ethynylferrocenyl)-benzene – a new member of the family of multiferrocenyl-functionalized cyclic systems

Abstract

The consecutive synthesis of 1,3,5-triferrocenyl-2,4,6-tris(ethynylferrocenyl)benzene (6c) is described using 1,3,5-Cl3-2,4,6-I3-C6 (2) as starting compound. Subsequent Sonogashira C,C cross-coupling of 2 with FcC≡CH (3) in the molar ratio of 1:4 afforded solely 1,3,5-Cl3-2,4,6-(FcC≡C)3-C6 (4c) (Fc = Fe(η(5)-C5H4)(η(5)-C5H5)). However, when 2 is reacted with 3 in a 1:3 ratio a mixture of 1,3,5-Cl3-2-(FcC≡C)-4,6-I2-C6 (4a) and 1,3,5-Cl3-2,4-(FcC≡C)2-6-I-C6 (4b) is obtained. Negishi C,C cross-coupling of 4c with FcZnCl (5) in the presence of catalytic amounts of [Pd(CH2C(CH3)2P(tC4H9)2)(μ-Cl)]2 gave 1,3-Cl2-5-Fc-2,4,6-(FcC≡C)3-C6 (6a), 1-Cl-3,5-Fc2-2,4,6-(FcC≡C)3-C6 (6b) and 1,3,5-Fc3-2,4,6-(FcC≡C)3-C6 (6c) of which 6b is the main product. Column chromatography allowed the separation of these organometallic species. The structures of 4a,b and 6a in the solid state were determined by single crystal X-ray diffractometry showing a π–π interacting dimer (4b) and a complex π–π pattern for 6a. The electrochemical properties of 4a–c and 6a–c were studied by cyclic voltammetry (=CV) and square wave voltammetry (=SWV). It was found that the FcC≡C-substituted benzenes 4a–c show only one reversible redox event, indicating a simultaneous oxidation of all ferrocenyl units, whereby 4c is most difficult to oxidise (4a, E°′1 = 190, ΔEp = 71; 4b, E°′1 = 195, ΔEp = 59; 4c, E°′1 = 390, ΔEp = 59 mV). In case of 4c, the oxidation states 4c(n+) (n = 2, 3) are destabilised by the partial negative charge of the electronegative chlorine atoms, which compensates the repulsive electrostatic Fc+–Fc+ interactions with attractive electrostatic Fc+–Cl(δ−) interactions. When ferrocenyl units are directly attached to the benzene C6 core, organometallic 6a shows three, 6b five and 6c six separated reversible waves highlighting that the Fc units can separately be oxidised. UV-Vis/NIR spectroscopy allowed to determine IVCT absorptions (=Inter Valence Charge Transfer) for 6c(n+) (n = 1, 2) (n = 1: νmax = 7860 cm(−1), εmax = 405 L mol(−1) cm(−1), Δν1/2 = 7070 cm(−1); n = 2: νmax = 9070 cm(−1), εmax = 620 L mol(−1) cm(−1), Δν1/2 = 8010 cm(−1)) classifying these mixed-valent species as weakly coupled class II systems according to Robin and Day, while for 6a,b only LMCT transitions (=ligand to metal charge transfer) could be detected.

Multiferrocenyl Cobalt‐Based Sandwich Compounds

“…The first and second redox processes in 3a and 6a – c are related to the butadiene‐ or cyclobutadienone‐bonded ferrocenyl groups, whereas the third and fourth redox events can be assigned to a subsequent oxidation of the FcC≡C ferrocenyl functionalities , . For 3a , a potential difference between the first two redox processes of Δ E °′ = 250 mV was found, which is much higher than the Δ E °′ values between the second and third as well as the third and fourth process (Table ).…”

“…The electrochemical properties of 3a , 5a , b , 5c , 6a – c , and 7 were studied by using cyclic voltammetry (CV) and square‐wave voltammetry (SWV) and by in situ UV/Vis/NIR spectroelectrochemistry ( 3a and 6a – c ) (Figures , , , , and S68–S70; Tables and ). The electrochemical measurements were performed under argon in dichloromethane solutions containing [ n Bu 4 N][B(C 6 F 5 ) 4 ] (0.1 mol L –1 ) as supporting electrolyte , , . The weak coordinating counterion [B(C 6 F 5 ) 4 ] – is known to stabilize highly charged species in solution.…”

“…The physical parameters have been determined by deconvolution using three Gaussian‐shaped curves (Figure , Table ). Monocationic 3a + shows two IVCT absorptions, one at 7400 cm –1 ( ε = 930 L mol –1 cm –1 , Δν̃ 1/2 = 5350 cm –1 ), which is probably related to electronic interactions between the two Fc/Fc + moieties across the cyclobutadiene unit, and a weaker absorption at 4800 cm –1 ( ε = 230 L mol –1 cm –1 , Δν̃ 1/2 = 2000 cm –1 ) attributed to an interaction pathway between the Fc + unit and the FcC≡C functionalities (Figure ) , , . Furthermore, a ligand‐to‐metal charge‐transfer band (LMCT) was observed at 3900 cm –1 ( ε = 580 L mol –1 cm –1 , Δν̃ 1/2 = 1200 cm –1 ).…”

“…Only one IVCT absorption was detected for 3a 2+ (Figure ) at 6100 cm –1 ( ε = 300 L mol –1 cm –1 , Δν̃ 1/2 = 3150 cm –1 ), attributed to the charge transfer from the Fc + to the FcC≡C functionalities. The shift to higher energy is the result of the lower electron density in the cyclobutadiene moiety induced by the two electron‐withdrawing ferrocenium groups , , . Due to the characteristics of those charge‐transfer excitations, including low intensity and the relatively high Δν̃ 1/2 , species 3a + and 3a 2+ can be classified as weakly coupled class II systems according to the Robin and Day classification .…”

Multi‐Ferrocenyl Aryl Ethers – Applying Nucleophilic Aromatic Substitution Reactions to Aryl Fluorides

“…This trend is also observed in 5e (90 mV, 195 mV) and 6c (93 mV, 126 mV). However, this differs for 6d (100 mV, 130 mV, 90 mV), probably due to an electrostatic compensation of the positive charge at the ferrocenyl groups by the partially negatively charged ortho ‐fluorine atoms . The smallest redox separation is observed in 6b (Δ E °′ = 48 mV), revealing small electrostatic interactions through the para positioning of the OFc moieties.…”